Acoustical imaging uses ultrasonic signals, generally with frequencies in the 1-10 MHz range, to provide images of adjacent objects by receipt of waves that are reflected from such objects or of waves that have been transmitted through or diffracted from such objects. One version of such imaging is two-dimensional echocardiography, which uses a reconstruction of objects from the echo waves reflected from each object in the path of the transmitted wave that is received by an array of receivers positioned adjacent to, but spaced apart from, the object.
Where a wave traveling in a first medium, having an acoustic impedance Z.sub.1 =.rho..sub.1 v.sub.1 (.rho..sub.1 and v.sub.1 are mass density and propagation velocity, respectively, in medium no. 1) encounters a second medium at normal incidence, the incident wave will be partly reflected from this surface with an amplitude reflection coefficient given by r=(Z.sub.2 -Z.sub.1)/(Z.sub.2 +Z.sub.1), where Z.sub.2 is the acoustic impedance of the second medium. This reflection will change with incidence angle. In the soft tissues of a human body, the reflection coefficient varies from -10 dB (between fat and muscle) to about -23 dB (between kidney and spleen). These correspond to low level reflections of less than 1% so that most of the acoustic energy is transmitted through the interface and is available for imaging structures that lie further from the transmitter. A relatively high magnitude reflection can take place at a bone/muscle interface, which has a reflection coefficient of about 40% (-4 dB). In this instance, only about half the energy is transmitted and available for imaging deeper structures.
Another problem in acoustic imaging of relatively soft structures such as body organs and tissues is that propagation of a wave in any particular body organ or tissue has an associated attenuation with a strongly frequency dependent attenuation coefficient. That is, as a wave propagates in such a medium, its intensity I diminishes from its initial value I.sub.0, as propagation distance z increases, according to the relation EQU I=I.sub.0 exp [-2.alpha.z], (1)
where the attenuation coefficient .alpha. increases approximately linearly with temporal frequency f of the wave: .alpha.(f)=.alpha..sub.0 +.alpha..sub.1 f with .alpha..sub.o being a constant approximately equal to 0.1 and .alpha..sub.1 being a scaling factor approximately equal to 1 dB/cm-MHz. Thus, for example, a three MHz acoustic wave that has traveled 20 cm through soft tissue has an intensity that is 60 dB (a factor of 10.sup.-6) below its initial intensity level; and if the intensity is increased to f=10 MHz, this acoustic beam would be 200 dB below its initial intensity level. For this reason, acoustic waves of lower frequency, of the order of 1-5 MHz, are used for imaging structures deep in the body and higher frequency acoustic waves, f=10 MHz, are used for imaging structures close to a surface within the body.
The velocity of propagation of a wave with a nominal frequency of 1-10 MHz within the body ranges from 1.41.times.10.sup.5 to 1.59.times.10.sup.5 cm/sec for various body organs and is not strongly frequency dependent in this range. An average value for wave propagation velocity v.sub.b in the human body of 1.54.times.10.sup.5 cm/sec is often used for modeling purposes. An exception to use of this average value is human bone, with a wave propagation velocity of 4.08.times.10.sup.5 cm/sec and with a characteristic impedance that is about 5 times that of the soft organs and tissues within the human body.
Where two or more adjacent organs or tissue interfaces are acoustically imaged within a human body, if these objects lie at different distances from the source or transmitter of the wave, the reflected waves will arrive at the receiver at different times and possibly from different directions relative to a center line that defines the orientation of the array of signal receivers. This has at least two consequences. First, the incoming wave from any one object may not be planar and may arrive from a direction that defines a non-zero incidence angle relative to the receiver array. Second, two incoming waves produced by two spaced apart objects will generally arrive at different times, with different incidence angles and with different shapes for the incoming waves. Other workers in this field often call for dynamical focussing, whereby each receiver in the receiver array is given a variable time delay that is commensurate with the direction from which a given incoming wave arrives. If two such waves are separated sufficiently in time, one set of time delays associated with an array of receivers can be replaced by a second set of time delays in the time interval between arrival of the first incoming wave and arrival of the second incoming wave. This is discussed by Brookner in "Phased Array Radar," Scientific American (January, 1985) pp. 94-102.
An example of this approach is disclosed in U.S. Pat. No. 4,116,229, issued to Pering for acoustic imaging apparatus. A time delay associated with a given receiver or transceiver or transducer is decomposed into a first large time delay contribution set by a tap on a master delay line and a second, smaller incremental time delay that is controlled by a set of controllable switches. The total time delay includes the first and second contributions to time delay and the incremental time delays can be changed at a predetermined time by use of the switches. A similar idea is disclosed in U.S. Pat. No. 4,140,022, issued to Maslak, in which focussing occurs by adjusting the phases of the waves. A mixer, in which the phase of the local oscillator is varied, effects the focussing phase variation in the signal.
Jones, in U.S. Pat. No. 3,869,693, discloses a beam scanner for a plane wave arriving at a non-zero incidence angle relative to a linear array (assumed vertical) of transducers. The transducers individually sense the arrival, at possibly different times, of the wave front. Each transducer is provided with a multi-component delay line that includes: (1) a first component that introduces a fixed time delay .DELTA.t.sub.1 that progressively increases as one moves along the linear array from the topmost transducer to the bottommost transducer; (2) a second component that introduces a variable time delay .DELTA.t'.sub.2, wherein the maximum extent of the variable range of .DELTA.t'.sub.2 progressively decreases as one moves along the linear array from the topmost transducer to the bottommost transducer, wherein the minimum extent of the variable range is zero, wherein .DELTA.t'.sub.2 is variable over its range in seven equal time increments; and (3) a third component that introduces a variable time delay .DELTA.t".sub.2, wherein the maximum extent of the variable range of .DELTA.t".sub.2 is a fractional 7/8ths of one of the seven equal increments of the respective value of t'.sub.2, wherein .DELTA.t".sub.2 is variable over its range in three increasingly large time increments, the first increment being 1/7th of its total range, the second increment adding another 2/7ths of its total range and the third increment adding the final 4/7ths of its total range. The second and third time delay components are used to provide a combined time delay, of amount given by the sum .DELTA.t.sub.c =M .DELTA.t'.sub.2 +(m.sub.1 /2+m.sub.2 /4+m.sub.3 /8) .DELTA.t".sub.2 (M=0,1,2, . . . , 7; m.sub.1, m.sub.2, m.sub.3 each=0 or 1 independently), for a signal arriving at a transducer. It appears that the combination time delay .DELTA.t.sub.c is to be combined with the fixed time delay .DELTA.t.sub.1 to obtain the net time delay introduced at a given transducer. The net time delay introduced at a given transducer is not continuous but has 64 discrete values, corresponding to the choices of the four integer coefficients M, m.sub.1, m.sub.2 and m.sub.3 of the combination time delay .DELTA.t.sub.c.
Time delay of a signal transmitted by a first transducer and received by a second transducer may be introduced by insertion of a piezoelectric element extending between the two transducers. In U.S. Pat. No. 3,537,039, issued to Schafft, an electrical field is applied transversely to control the time delay of torsional vibrations of the piezoelectric material that carry the signal from the first transducer to the second transducer.
In U.S. Pat. No. 4,342,971, issued to Alter, application of a transverse electrical field alters the length of the piezoelectric element inserted between the first and second transducers and provides a controllable time delay for a signal sent between the two transducers. This approach is also disclosed in U.S. Pat. No. 4,401,956, issued to Joshi. In all these patents, the variable time delay introduced by the electrical field applied to the piezoelectric element appears to be at most a few percent of the time delay associated with the piezoelectric element with no electrical field applied.
Where the means for providing time delay are explicitly disclosed in the prior art, these devices appear to be rather large and electronically complex so that only a modest number of receivers can be provided with variable time delays. Often, the means of providing such variable time delay is not disclosed.
In order to provide adequate sampling of an incoming wave for any incidence angle from 0.degree. to 90.degree., the receivers in the array should be spaced apart by no more than one half the wave length, .lambda., corresponding to the central frequency of the incoming wave, according to the Nyquist theory of (under) sampling. If the central frequency is chosen to be f=5 MHz and a propagation velocity of v=1.54.times.10.sup.5 is assumed, the receiver-to-receiver spacing should be .lambda./2=v/2f=154 .mu.m or smaller. A one-dimensional array having a modest number N=100 such receivers would require that all these receivers be approximately linearly aligned and uniformly spaced along a distance of about 1.5 cm. For a two-dimensional array with N=10.sup.4 such receivers, this many receivers would have to be positioned in a rectangle or similar figure of area approximately 2.25 cm.sup.2. It is unlikely that this could be done for the receivers with variable time delay discussed in the previous literature.
What is needed is acoustic imaging apparatus that will allow introduction of controllable, variable time delay in the signal produced at each receiver and will allow a large number of such receivers to be positioned in a very small length or small area representing the receiver array. Such receivers should, preferably, also allow the time delay at any one receiver to be changed in a time of the order of microseconds or less in order to provide adequate discrimination between two incoming acoustic waves corresponding to two different objects to be imaged by the apparatus.